The invention relates to an internal combustion engine run on Otto fuel which may be compression ignition operated in a first operational range and spark ignition operated in a second operational range, with means for adjusting valve timing of at least one intake valve and of at least one exhaust valve, with at least one ignition equipment and at least one direct injection equipment in each cylinder, and to a method for operating said internal combustion engine.
The combustion of an auto-ignited lean fuel-air mixture has the advantage that extremely low NOx and soot emissions are obtained on account of the homogeneous distribution of concentration and temperature. This process is known as HCCI combustion (Homogeneous Charge Compression Ignition). HCCI combustion results in low NOx emissions, which is due to the fact that combustion is initiated at multiple ignition sites, the temperature of the combustion process being relatively low as a result thereof. For HCCI combustion, gasoline presents great advantages over diesel fuel on account of its low autoignition quality and the lower boiling range of between approximately 30xc2x0 C. and 190xc2x0 C. The compression ratio may be raised to values similar to those in a diesel engine of about 15 to 17. Since the precise time of ignition can be fixed as desired just before top dead center only when the effective medium pressure is low, the effective medium pressure achievable in HCCI combustion is disadvantageously limited to the part load range as may be gathered from the publication entitled xe2x80x9cAn Experimental Study on Premixed-Charge Compression Ignition Gasoline Enginexe2x80x9d, Taro Aoyama et al., SAE Paper No. 960081.
DE 199 27 479 A1 describes a method for operating an engine run on gasoline in which the internal combustion engine is operated in the homogeneous charge compression ignition mode when the effective medium pressure is below a predetermined limit and in the spark ignition mode when said effective medium pressure is above said limit. In this way, all the advantages of HCCI combustion can be made use of without the disadvantages thereof.
HCCI combustion primarily intends to cover the low load and the part load range, whereas spark ignition operation is used for higher part load and full load. In the dynamic operation mode of an internal combustion engine it is absolutely necessary to transition the engine from one operating mode to the other without generating serious losses in torque delivery in the transition phases.
EP 1 085 192 A2 discloses an internal combustion engine which is operated in the homogeneous charge compression ignition mode in the medium part load range and which is operated in the homogeneous charge spark ignition mode in the upper part load range and at full load, and in the lower part load range as well. In the transition range from the spark ignition mode to the compression ignition mode, the quantity of recirculated exhaust is increased in order to ensure safe autoignition. Conversely, on transitioning from the compression ignition mode to the spark ignition mode of operation, the quantity of recirculated exhaust is reduced in time to prevent knocking.
DE 196 02 013 A1 discloses a lift changing device by means of which the valve lift may be varied. Zero lift is also possible.
EP 0 156 996 A1 describes a cam actuated valve timing system with electrohydraulic valve lifting systems that are driven by a microprocessor. The valve lift may be configured in a largely flexible manner.
The U.S. Pat. No. 5,647,312 A discloses a four-stroke internal combustion engine with hybrid control provided with a main intake valve and with an additional intake valve. The cam actuated main intake valve can be actuated into an OFF position by way of a control element provided in the transmission path between intake cam and valve stem. The valve lift of the additional intake valve can be varied through an actuating device as a function of the engine load.
DE 43 17 607 A1 and AT 4.872 U1 describe variable valve gear trains for lift valves, each of them being provided with a hydraulic actuating member arranged in a cup-shaped tappet. Said actuating member permits to produce a hydraulic additional lift in addition to the mechanical lift determined by the cam. In particular the valve gear train presented in the utility model application is suited to produce the hydraulic lift independent of the position of the camshaft, that is to say also for actuating, in the intake phase, an exhaust valve for the purpose of recirculating residual gas for example.
Known internal combustion engines that run on Otto fuel and are operated both in the compression ignition mode and in the spark ignition mode are equipped with an equipment for realizing a fully variable valve gear train and a combustion-guided engine timing system in order to achieve optimal combustion control and, as a result thereof, high efficiency through fast combustion as well as lowest possible NOx and soot emissions. The disadvantages thereof however are the complexity of construction and the considerable cost involved.
It is the object of the invention to develop an internal combustion engine that can be securely operated in the simplest possible way on Otto fuel both in a compression ignition and in a spark ignition mode.
This is achieved, in accordance with the invention, in that there is provided at least one intake camshaft for actuating the intake valves and at least one exhaust camshaft for actuating the exhaust valves, that at least one intake valve in each cylinder may be moved between a first and a second lift position by a device for changing the valve lift, and that at least one exhaust valve is actuatable by a residual gas recirculation system during the intake lift. Accordingly, the internal combustion engine is provided with a simple valve gear train with one intake camshaft and with one exhaust camshaft. The lift change between the first, small lift and the second, large lift for at least one intake valve is effected by way of a simple device for changing the valve lift that may be mechanically, hydraulically or electrically actuatable. The device for changing the valve lift can for example be realised by providing in each intake valve two intake cams with varying lift pattern.
In an internal combustion engine with at least two intake valves in each cylinder, there is preferably provided that one intake valve is changeable between the first and the second lift by means of the device for changing the valve lift and that the other intake valve may be actuated into an OFF position by means of a valve cut-off device. Alternatively, there may also be provided, in an internal combustion engine with two separate intake manifolds, that the two intake valves may be moved by the device for changing the valve lift between a first lift and a second lift and that one intake manifold in each cylinder may be cut off by way of a control apparatus.
In the first operational range of the engine, the lift of at least one of the intake valves is reduced from a second lift position to a first lift position. In the second operational range of the engine, the lift of this intake valve is again increased from the first to the second lift position. The cylinder filling is thereby controlled by the throttle. There may also be provided that, in a transition range between the first and the second operational range, the internal combustion engine is operated with an air-fuel ratio xcex=1, the lift of at least one intake valve being changed to the first lift position.
In order to stabilize combustion more specifically during the transition from the spark ignition mode to the compression ignition mode, the opening time in the first operational range of the engine can temporarily be displaced toward the top dead center of ignition. Turbulence in the combustion chamber is increased as a result thereof, which advances combustion. This can be achieved when the intake camshaft is rotatable by an intake phase changer.
In order to achieve the increase in temperature of the charge which is needed for safe autoignition, there is provided that, in the first operational range of the engine, at least one exhaust valve in each cylinder is opened during the intake stroke, the residual gas recirculation being controlled at least through the phase position of said second exhaust lift, the recirculation system for the residual gas being provided with an exhaust phase changer for rotating the exhaust camshaft for the purpose of performing said step of the process. A particularly flexible control of the combustion process may be achieved, when the second exhaust valve is hydraulically or electrically actuated during the intake stroke, phase position and valve opening duration being preferably varied. It is particularly advantageous to adjust duration and/or position of the second exhaust lift for each cylinder individually, the combustion situation and the rate of combustion of the respective one of the cylinders being controlled by said individual adjustment of duration and/or position of the second exhaust lift for each cylinder. In adjusting duration and/or position of the second exhaust lift the stability of combustion may be particularly improved when the operation mode is unsteady. In the spark ignition transition range, the internal residual gas recirculation system permits to achieve unthrottled operation, which will result in a reduction in consumption.
As compared to a fully variable valve gear train, the construction proposed and the method described are simple and inexpensive. It allows compression ignition operation at part load thanks to high internal residual gas.
Since conventionally the gas exchange valves are mechanically actuated, the oil temperature has no effect on the actuation of the valves. This has the advantage that cold start may be carried out in a conventional way in the spark ignition mode.
The mechanical valve actuation permits to achieve high RPMs and high loads with a conventional large lift, which causes the least possible friction losses and makes unrestricted full load operation possible.
Particularly low fuel consumption and very low emissions may be achieved by the lift change in the spark ignition mode at low and medium part load.